Mechanism-related patterns of in vitro sensitivity have been recognized since very early in the development phase of the current NCI primary antitumor drug screen. Development of the "Compare" programs has allowed routine comparison of patterns produced by unknown compounds with the standard agent database (which contains prototype agents representing all known mechanistic classes). In several instances this has allowed insight into the potential mechanism of action of the unknown compound. In this project we have begun to dissect the molecular basis for the mechanism-related patterns. Because the multidrug-resistant (MDR) phenotype affects several mechanistic classes, we initiated work in this area. Several classic MDR-1 type cell lines were identified in the screening panels. Analysis of DNA topoisomerase II isoform mRNA using a very sensitive RNA PCR method failed to demonstrate any correlation with in vitro drug sensitivity even when MDR-1 expressing cell lines were excluded from the analysis. We have more recently extended our MDR studies to address genes in addition to the classic MDR-1 gene. Evaluation of expression of MRP and LRP-56 at the protein level has shown that many tumor cell lines concurrently express multiple MDR mechanisms. Among these three MDR mechanisms, available data suggest that LRP-56 may be most specifically associated with the drug resistant phenotype. Analysis of factors potentially related to sensitivity to alkylating agents has shown that expression of O6-methylguanine DNA methyltransferase (MGMT) and growth kinetics are major determinants, particularly in relation to chlorethylating agents. A multiparameter analysis of these data as well as mRNA data for two genes related to glutathione is currently in progress. Through evaluation of additional genes we hope to gain a more complete understanding of the drug sensitivity patterns observed in the screening data.